A particular area of application for the present invention is in transistor amplifiers used in exhaust gas detectors in a motor vehicle. In a known system of this type, disclosed in DT AS No. 21 08 550, the effective input impedance of the transistor amplifier is increased by connecting a reference transistor identical to the amplifier transistor into the collector circuit of the latter. The base current drawn by the reference transistor is applied by means of a so-called "mirror" circuit to the base of the amplifier transistor thereby decreasing the load on the signal source by a factor (B+1) where B is the current amplification of the amplifier transistor (and therefore of the reference transistor).
The main disadvantage of this known transistor amplifier is that the input voltage may not be smaller than the minimum voltage required for causing the emitter-base diode of the transistor in the "mirror" circuit which conducts the base current to be conductive. If the input voltage is less than that, the amplifier does not respond at all. The input voltage must be even larger than the above-mentioned amount if the amplification of the input signal is to be distortionless. This greatly decreases the number of applications for the known transistor amplifier.